High density dual track redundant recording system



Aug. 2, 1966 A. GAB'OR 3,264,623

HIGH DENSITY DUAL TRACK REDUNDANT RECORDING SYSTEM orig nal med May 5, 1960. 4 Sheets-Sheet 1 l v 3 7 x ufk fi 6 mm FLOP QCURRENT 3; CELL DELAY 7 FIG I HEAD j l6 we 20 E,' fl \INTEGRATIONPV v A I 2| 23 24 25 men PASS RC FILTER 7 CL'PPER M'XER COUPLING I31 5 174m? A INTEGRATION}\ I9 22 V CLOCK 9 7 1 L AND OUT [29 NO I MMV T002511 m4 7 32 ,MMv w -5 33" r34 35 'NVERTER 3e mp0 AND 1 our f 39 N02 @Z OAMZCW.

ATTORNEY Aug. 2, 1966 A. GABOR 3264,23

HIGH DENSITY DUAL TRACK REDUNDANT RECORDING SYSTEM Original Filed May 3. 1960 4 Sheets-Sheet 2 0 DATA TO BE REco DED o o o I I I; 0 o 0 H CLOCK INPUT To E RECORD C'RCUT JLJLJLJLJLJLJLJLJLJLJL DATA INPUT T0 REcoRD F cIRcuIT (DELAYED BY ME LAA L 5 I-IALF 0F GIT cELL TIME) 42 g I G LINE E AND LINE F JLJLJLJWWWLJLJLJWL gg, RECORD cuRRENT wAvE FORM 2 H (OUTPUT OF E FLIP- LFLF'LF F FLOP DRIvEN BY LINE G) 1 J I E I PLAYBACK wA /EFoRM a O m J 1 AFTER LINEAR SHAPING f 3 BY FILTER Z i K .I AFTER CLIPPING LIT If 53 L K AFTER INTEGRATION L/LA/ 5 M K AFTER INvERsIoN U L o N M AFTER INTEGRATION /L/L./ AL/IL g I) 0 LINE L+ LINE N /l/]// I/V/V///1/ML P LINE DAFTER R'C ALJQQMNMWLJLJUJJL COUPLING A GATE sIGNAL DERIVED .11 1'1. 1'1 Q FROM UNEP FLJ'L .FL .FhfLJ'LfL i g a O R CLOCK ouTPuT;LINE JLJLJLJLJLJLJLJLJLJLJL 55g P GATED WITH LINE 0 'g 5 DATA OUTPUT- LINE igqg P GATED wITI'I LINE 0 k M m A INvERTED INVENTOR ANDREW GAEoR FIG. 2 BY 4 ATTDRNEY Aug. 2, 1966 l I I GABOR 3,254,623

. HIGHDENSITY DUAL TRACK REDUNDANT RECORDING SYSTEM Origir 1al Filed May 3. 1960 4 Sheets-Sheet 3 OUTPUT 232% waver-0am 'ILFU'L A I 4 FIG. 4

| a o l o o I H4 one CELL FIG. 5

INVENTOR.

ANDREW GABOR ATTORNEY AugPzj-y1966- I 4 A.GABOR 3,264,623

HIGH DENSITY DUAL TRACK REDUNDANT RECORDING SYSTEM Original riiedua 5,1960 I 4-$heets$heet 4 50, 52 54 F 5g 58 so s2 RECORD Q TRACK PLAYBACK s4 65 mPur AMP I AMP l.) /O-0 OUT 49 RECORD TRACK PLAYBACK AMP 2 7 AMP 5| 53 -55 l 57 s9 -s1 s3 FIG.6

- HEAD 69, 11 73 RECORD J TRACK Z 79 AMP v' I PLAYBACK J AMP /ouT RECORD. TRACK AMP 2 A 10 7 74/. 7e

,HEAD 85,87 89 v 82 83 TRAC 'J PLAYBACK 93 INPUT l' AMP. 94 9 RECORD OUT AMP TRACK PLAYBACK v 2 v AMP- a4, as/ as 90 92 HEAD IOI) Z7 99 TRIICK 0205 I06) I07 INPUT REC RD PLAYBACK v D AMP AMP. Q)OUT TRACK 96 98 f 2 I00 102 I04,

FIG. 9; l DROP OUT FOR SINGLE TRACK l eo-m.s-+ z '5 5% Q ZOMILS ZOMILS K" .2: ml- 3 0: i: 2 E Y gg, '3 TRACKFI TRACK *2 O F FIG.

uwswron 3. ANDREW GABOR CHANNEL. SPACING AB FIG.|O a/L kc/7% ATTORNEY United States Patent 3,264,623 HIGH DENSITY DUAL TRACK REDUNDANT RECORDING SYSTEM Andrew Gahor, Huntington, N.Y., assignor to Potter Instrument Co. Inc, Plainview, N.Y., a corporation of New York Original application May 3, 1960, Ser. No. 26,538, now Patent No. 3,217,329, dated Nov. 9, 1965. Divided and this application Apr. 29, 1965, Ser. No. 451,915

Claims. (Cl. Mil-174.1)

This is a division of Serial No. 26,538 filed May 3, 1960 now Patent No. 3,217,329 and entitled High Density Recording System.

The present invention concerns magnetic recording and, in particular, means for high density recording with selfclocking characteristics.

In digital computer memory systems, as well as in other applications, magnetic recording on plastic base tapes has undergone wide application. In most of these systems and applications it is important to record as much information per unit of length as possible. The recording is in the form of bits of two kinds, commonly designated as yes and no or 1 and 0. Broadly two types of recording may be utilized; return to zero (RZ) or nonreturn to zero (NRZ).

In the former, the tape receives signals representing the bits separated by intervals of zero signal which produce spaces of zero magnetic flux between bits. In the latter, there are no zero signal intervals between the bits of information recorded on the tape.

In order to record more information per unit length on a magnetic tape than is possible with a single track, it is common torecord a number of parallel tracks on the tape. In order to conserve space, it is also common to provide a single synchronizing track for several parallel record tracks.

Simple non-return to zero recording runs into serious difficulties when the density of recording is increased to a point Where the response of the system exhibits nonlinear frequency distortion within the operating range. This tultimately places an upper limit on the bit density which may be recorded and recovered. This difficulty may be overcome by a method of high density recording with a contiguous ,waveform involving at least one fiux reversal per bit cell set forth in US. Patent No. 2,853,357, page 5.

According to the present invention, a signal is utilized in recording information which is a modification of the signal and method set forth in the above referred to patent. This provides a high density, self-clocking signal. In addition, it has been found that when two tracks are recorded separated by a distance of the order of sixty mils that the optimum condition is provided of minimum drop out due to imperfections of the recording surface and drop out due to time displacement between the tracks.

The playback of this high density, high reliability recording is carried out in accordance with one of four possible methods of combining the readings of the two record tracks to form a redundant result. First, the system may be practiced by using separate record amplifiers, separate playback amplifiers and by combining (OR gating) the outputs of the playback amplifiers.

Second, the recording may be done through separate channels and the playback by combining the outputs of two playback heads. Third, recording may be done by feeding int-o two recording tracks in combination and playing back through separate amplifiers. Fourth, recording may be done by feeding into combined recording tracks and playing back from the combined tracks. In all of the above, reversing the polarity of signal between the two tracks reduces the resultant noise to signal level.

The recording, according to the present invention, con- "ice sists in a system utilizing a self-clocking, non-return-tozero signal. Clock pulses marking the bit cell boundaries are fed to the record amplifier continuously regardless of data bit combination. Data pulses consisting of a pulse for a l and an absence of pulse for a 0 are delayed so as to be situated at the center of the cell.

After mixing clock and data pulses, a current transition is generated for each pulse by means of a flip-flop or equivalent circuit. The resulting signal is the recording waveform characterized by a current transition at each cell boundary. It has been found that this method of recording signal generation provides a signal which prevents pulse crowding and which is selfclocking. When two such signals are utilized and are recorded at optimum spacing on the recording tape, a system is provided having maximum density per inch capabilities combined with minimum drop-out or loss of information.

Accordingly, one object of the present invention is to provide a method of and means for high reliability, high density digital data recording.

Another object is to provide a high density recording which is self-clocking.

Still another object is to minimize drop-out in a high density data recording system.

A further object is to provide a system with maximum reliability in the signal recovered from the tape recording.

A still further object is to provide a simple and effective method of and means for starting the recording according to the present invention which is not ambiguous.

Another object is to provide redundant recordings at high density and high degree of reliability.

These and other objects of the present invention will be apparent from the detailed description of the invention when taken in connection with the various figures of the drawing.

In the drawings:

FIG. 1 shows a block diagram of a portion of a recording system in accordance with the present invention.

FIG. 2 shows a series of signal waveforms utilized in a system according to the present invention.

FIG. 3 shows a block diagram of another portion of a recording system in accordance with the present invention.

FIG. 4 shows circuit details of an RC integrator suitable for use in systems according to the present invention.

FIG. 5 shows comparison between the recording waveform of the present invention and the waveform of the reference patent.

FIG. 6 ShOWs a block diagram of one way in which playback of a redundant recording may be carried out in accordance with the present invention.

FIGS. 7, 8 and 9 show block diagrams of additional Ways in which redundant playback in accordance with the present invention may be carried out.

FIG. 10 shows curve-s of signal loss in magnetic recording useful in explaining the operation of the present invention.

FIG. 11 shows typical track Width and spacing in accordance with the present invention.

FIG. 1 shows how clock input signals A are applied to input terminal 1, information input signals B are applied to input terminal 2. The information signals applied at 2 are delayed one-half cell period in delay circuit 4 and are applied over line 5 to mixer 3. Clock signals A applied at 1 are also fed to mixer 3. The mixed clock and delayed information signals are applied to flip-flop '7 over line 6. The flip-flop output appears at 8 for controlling the recorded signal in the form shown at C.

FIG. 2 shows a series of waveforms typical of the signals for a recording and playback system according to the present invention. Line D indicates a series of 0s and 1s to be recorded. Line B shows the clock input signal to the record circuit. Line F shows the waveform of the signal generated by the data to be recorded delayed by one-half of a bit cell time. Line G shows the sum of signals of lines E and F.

Line H shows the record current waveform which is the output of the flip-flop as shown in FIG. 1. This series of waveforms up to'this point are typical of a system according to the present invention up to the generation of recording current. Details of the system prior to parts shown in FIG. 1 have not been given since it is well known in the art how signals are generated up to this point.

The data signals and clock signals at 1 and 2 of FIG. 1 are assumed to have been generated so as to be in time phase. This will be the usual situation where data is released for recording by the clock signals. The delay device 4 for delaying the input information by one-half cell I through a high pass filter to provide the waveform shown at J. picks up the playback waveform, passes it to linear amplifier 11 over line 10, which, in turn, passes the amplified signal to high pass filter 13 over line 12.

This high pass filter 13 may be a simple two section RC filter. The amplified signal is passed to a clipper 15 over line 14. The clipper 14 is one which clips'the. signal both top and bottom close to the base line pnoducing the waveform K of FIG. 2 which will be seen to be a reproduction of the recording current waveform of line H. The next step is to recover the waveform of line G which may be accomplished by means of a simple RC coupling filter and a full wave rectifier but, however, may be accomplished to advantage in a less direct manner.

The square waves K are fed over line 16 to integrator 18 (see FIG. 4) to line 20 and mixer 21. After this integration, the waveform of L in FIG. 2 is provided. Also, in FIG. 1, the waveform K is inverted in inverter 17 and the resulting inverted signals fed to mixer 21 over line 22. The inverted waveform is shown at M and the integrated inverted waveform at N. The two waveforms L and N mixed in mixer 21 appear on line 23 as shown at O.

This waveform in turn is passed through RC coupling 24 appearing on line 25 as shown at O and P, respectively. The waveform P on line 25 is applied to AND gate No. 1 (27) over line 26 and AND gate No.2 (38). The output of gate 27 over line 28 is applied to a pulse generator monostable multivibrator 30 over line 29 which has a pulse ON period of 0.25 of the repetition period. The output of this pulse generator is applied over line 31 to pulse generator 32 which has a pulse ON period of 0.5 of the repetition rate and is triggered by the trailing edge of the pulse from 30.

Hence, the output of pulse generator monostable Inultivibrator 32 will be one-half period pulses delayed one quarter period as shown at Q. These pulses applied over line 34 to inverter 35,'the output of which is applied over line 36 to AND gate 27, inhibit those portions of the signal of line P Which are not clock pulses. Therefore the output over line 28 to point 40 will be made up of clock pulses only. The output of pulse generator 32 over line 33 when applied to AND gate 38 serves to inhibit all clock pulses and pass all signal pulses so that its output over line 33 to point 41 consists in information only as shown at S. At R is shown a representation of the clock pulses provided at point 40.

FIG. 4 shows an integrating circuit suitable for use in a system according to the present invention. This inte- This process is illustrated in FIG. 3 where head 9 grator includes a transistor having an emitter 44 connected to ground G, a base 43 connected to an input point 42 and a collector 45 connected through a load resistor 46 to a source of bias indicated as B. When a square wave as illustrated by the Input Waveform is applied to input point 42, output current in the transistor collector circuit charges capacitor 4'7-v through resistor 46 producing an integrated output at 48 having the form illustrated by the Output Waveform.

FIG; 5 shows a comparison between recording current waveforms of recording in accordance with US. Patent No. 2,853,357 on line T and the present invention on line U.

FIG. 6 shows a block diagram of a redundant playback of one form in accordance .with the present invention. Input signalsto be recorded are applied to the common input point 49 and divide over lines 50 rand 51 to recording amplifiers 52 .and 53, respectively. The outputs of these two recording amplifiers are applied, to recording heads 56 and 57 over lines 54 and'55 for recording on two independent tracks on the magnetic tape (not shown). Thus, information is recorded on magnetic tape utilizing two independent recording amplifier-sand two independent recording heads.

To play back the information thus recorded the same heads 56 and 57 are utilized to pick up the information from the tape and to feed it over lines-58 and 59 to playback amplifiers 60 and 61. The outputs of amplifiers 62 and 63 are applied to an OR circuit 64 over lines 62 and 63 to produce an output at output point 65. This output will then be the signal from head 56 or 57' so that if a defect in the tape causes any information to be missing from the output of one head, it will be present fromthe output of the other head and, at the output point, complete data will be presented.

FIG. 11 shows a typical recording pattern on magnetic FIG. 10 illustrates the application'of the redundant recording in accordance with the present invention .to magnetic recording genera1ly. The curve labeled Drop Out shows how the loss of recovered signal due to imperfections in the tape decreases as the spacing between the two utilized channels is increased. ,This shows'a large decrease in drop out-as thechannel. spacing is increased upto a pointafterwhich the gain in reliability falls off.

Another curve labeled Signal Loss represents the way in which the loss in signal increases with channel spacing due to increasing inter-channel time displacement. It will be seen that as the .drop out rate due to (defects decreases with channel spacing, the'signal loss increases. The Optimum Track Separation can be found where the drop out has decreased to a low level. and the signal loss has just started to. increase rapidly. As was-stated above, in one particular case the optimum condition was: substantially that in whichrthe channels were 20 mils Wide separated by 60 mils center to center..

up headsfor the recorded signals apply the picked up signals to a single playback amplifier 78 over lines and 76 combining in line. 77. The combined output then appears at output point'79.

FIG. 8 shows still another combination for providing the redundant recording and playback in which input signals are applied to point 80 and thus to a single recording amplifier 81. The output of the recording amplifier over line 82 is divided over lines 83 and 84 and applied to heads 85 and 86, respectively, providing the two record tracks on the magnetic tape (not shown). Heads 85 and 86 in playing back the redundant signals apply signals to amplifiers 89 and 90 over lines 87 and 88, respectively. The outputs of the two amplifiers over lines 91 and 92 are combined in the OR circuit 93 to provide an error free output over line 94 to output point 95.

FIG. 9 shows still another form of the redundant recording and playback system in which input signals at point 96 are applied to a single recording amplifier 97. The output of amplifier 97 over line 98 is divided over lines 99 and 100 and applied to heads 101 and 102, respectively, for recording two tracks on the magnetic tape (not shown). In playing back, the two heads 101 and 102 pick up signals from the record tracks and apply them over lines 103 and 104 combining in line 105 to the single playback amplifier 106. The error free output thus produced is applied to output point 107.

What is claimed is:

1. In a digital information recording and recovery system, the combination of, means for deriving self-clocking information signals from a prerecorded dual-track redundant magnetic tape wherein said redundant tracks are spaced by a predetermined amount at which the combined signal loss due to imperfections in the tape and due to inter-channel time displacement is substantially at a minimum, means for deriving from said signals a series of pulses representing composite information and clock signals, and means for separating said composite signals into separate clock and information signals.

2. In a digital information recording and recovery system, the combination of, means for picking up prerecorded signals from two tracks on a magnetic tape wherein said tracks are spaced by a predetermined amount at which the combined signal loss due to imperfections in the tape and due to inter-channel time displacement is substantially at a minimum, means for combining said picked up signals to form a substantially skip-free composite signal, means deriving from said composite signal a series of pulses of combined clock and data signals, and means for separating said combined clock and data signals into data signals and clock signals free from data signals.

3. In a digital information recording and recovery system, the combination of, a source of clock pulses, a source of information pulses synchronized with said clock pulses, means for mixing said clock and information pulses, means for generating recording current in accordance with said mixed pulses, dual-track redundant recording means for recording two tracks on magnetic tape spaced by a predetermined amount at which the combined signal loss due to imperfections in the tape and due to inter-channel time displacement is substantially at a minimum, means for supplying both of said dual-track recording means with said recording current, and means for deriving selfclocking information signals from said two tracks on said magnetic tape.

4. In a digital information recording and recovery system, the combination of, a source of clock pulses, a source of information pulses synchronized with said clock pulses, means for mixing said clock and information pulses, means for generating recording current in accordance with said mixed pulses, dual-track redundant recording means for recording two tracks on magnetic tape spaced by a predetermined amount at which the combined signal loss due to imperfections in the tape and due to inter-channel time displacement is substantially at a minimum, means for supplying both of said dual-track recording means with said recording current, means for deriving self-clocking information signals from said two tracks on said magnetic tape, means for deriving from said signals a series of pulses representing composite information and clock signals, and means for separating said composite signals into separate clock and information signals.

5. In a high density magnetic tape recording and recovery system, the combination of, a source of clock pulses, a source of information pulses, means defining a dual-track information channel wherein said tracks are spaced by a predetermined amount at which the combined signal loss due to imperfections in the tape and due to inter-channel time displacement is substantially at a minimum, means for delaying said information pulses by a period of time substantially equal to one-half the time between adjacent clock pulses, means for mixing the clock pulses and the delayed information pulses, means for generating recording current under control of said mixed pulses including means for recording the mixed pulses in both tracks of said channel, and means for picking up recordedmixed pulses from said dual-track information channel.

6. In a high density magnetic tape recording and recovery system, the combination of, a source of clock pulses, a source of information pulses, means defining a dual-track information channel with the tracks separated by a predetermined amount at which the combined signal loss due to imperfections in the tape and due to interchannel time displacement is substantially at a minimum, means for delaying said information pulses by a period of time substantially equal to one-half the period of said clock pulses, means for mixing said clock pulses and said delayed pulses to form a composite series of pulses, means for generating recording current under control of said mixed pulses including means for recording the mixed pulses in both tracks of said channel, means for deriving self-clocking information signals from said tracks of said channel, means for deriving from said signals a series of pulses representing composite information and clock signals, and means for separating said composite signals into separate clock and information signals.

7. In a digital information recording and recovery system as set forth in claim 6 wherein said predetermined amount is in the order of sixty mils.

8. In a digital information recording and recovery system, the combination of, a source of clock pulses, a source of information pulses synchronized with said clock pulses and of the type in which a pulse in a cell bit interval of time represents a 1 and an absence of a pulse in a cell bit interval of time represents a 0, means defining a dual-track information channel with tracks separated by a predetermined amount at which the combined signal loss due to imperfections in' the tape and due to interchannel time displacement is substantially at a minimum, means for displacing pulses from one of said pulses with respect to the pulses of the other of said sources by substantially one-half of a cell bit of an interval to place said information pulses substantially mid-way between said clock pulses, means for mixing said displaced information and clock pulses to form a composite series of pulses, means for generating recording current of two values switched from one of said values to the other by each pulse in composite series of pulses including means for recording the mixed pulses in both tracks of said channel, and means for deriving self-clocking information signals from each track of said channel.

9. In a digital information recording and recovery system the combination of means for deriving self-clocking information signals from a prerecorded dual-track redundant magnetic tape wherein the redundant tracks are spaced by an amount at which the signal loss due to imperfections in the tape is approximately equal to the signal loss due to inter-channel time displacement, means for deriving from said signals a series of pulses representing composite information and clock signals, and

means for separating said composite signals into separate References Cited bythe Examiner clock and information SigHE IIS. UNITED STATES PATENTS 10. In a digital information recording and recovery systerm the combination of, means for deriving self-clocking 2,813,259 11/1957 Burkhart 340174-1 information signals from a prerecorded dual-track re- FOREIGN PATENTS dundant magnetic tape wherein said redundant tracks are spaced by a predetermined amount on the order of sixty 792294 3/1958 Great Bntam mils, means for deriving from said signals a series of BERNARD KONICK, Primary Examiner. pulses representing composite information and clock v pulses, and means for separating said composite signals 10 IRVING SRAGOW Examine into separate clock and information signals. A. I.-NEUSTADT, Assistant Examiner. 

1. IN A DIGITAL INFORMATION RECORDING AND RECOVERY SYSTEM, THE COMBINATION OF, MEANS FOR DERIVING SELF-CLOCKING INFORMATION SIGNALS FROM A PRERECORDED DUAL-TRACK REDUNDANT MAGNETIC TAP WHEREIN SAID REDUNDANT TRACKS ARE SPACED BY A PREDETERMINED AMOUNT AT WHICH THE COMBINED SIGNAL LOSS DUE TO IMPERFECTIONS IN THE TAPE AND DUE TO INTER-CHANNEL TIME DISPLACEMENT IS SUBSTANTIALLY AT A MINIMUM, MEANS FOR DERIVING FROM SAID SIGNALS A SERIES OF PULSES REPRESENTING COMPOSITE INFORMATION AND 